1. Field of the Invention
This invention relates to an electric motor in which the stator is accommodated in a metal casing, and more particularly to an assembly structure which improves the motor characteristic thereof.
2. Related Art
As shown in FIG. 4, in a stepping motor 1, its body includes: a bottomed-cylinder-shaped casing 2 of metal; a stator 3 accommodated in the casing 2; and a rotor 4 which is inserted into a circular opening 3a formed in the stator 3. The motor body is mounted through a bracket 5 on a variety of devices. The stator 3 is formed by uniting ring-shaped stator yokes 6, 6 and 6 through resin bobbins 7 and 7 on which wires are wound.
As shown in FIG. 3, the inside diameter L2 of the casing 2 accommodating the stator 3 is lightly larger than the outside diameter L3 of the stator 3 so that the stator can be mechanically inserted into the casing 2. In the case where the difference (L2-L3) is constant, no problem is involved in the assembling of the stator and the casing. However, if the difference, being fluctuated, is large, then the stator 3 is not in close contact with casing 2, and the assembling of them involves a problem. In FIG. 3, reference numeral 8 designates wires wound on the bobbins of the stator 3.
In the case where the stator 3 is not in close contact with the casing 2, the air gap is formed between them to break the magnetic circuit, so that the torque is decreased, the self start frequency characteristic is lowered; that is, the motor characteristic is lowered.